Physical vapor deposition by sputtering is a well known process that has found widespread application in the fabrication of integrated circuit semiconductor devices. In semiconductor device fabrication, a large number of integrated circuit devices are normally formed on a thin, generally circular semiconductor substrate referred to as a wafer. Integrated circuit device fabrication involves a large number of processing steps, with sputtering typically being used to provide metallization layers and interconnects between device layers. Most commonly, sputtered aluminum is the material used for these purposes. Modern semiconductor processing has also seen the increased use of sputtered tungsten, tungsten silicide, titanium, titanium nitride and other films.
A magnetron sputtering source is capable of high rate sputtering and represents an enormous improvement over devices forming thin films based on diode sputtering or evaporative techniques. Magnetron sputtering sources are routinely used by the semiconductor processing industry to coat semiconductor wafers during the manufacture of integrated circuits.
In magnetron sputtering a plasma is formed in a low pressure inert gas by the application of a suitable voltage. The plasma is confined to a region near a sputter target, which is made of the material to be sputtered and which usually serves as the cathode of the system. A magnetic field, typically having field lines which loop through the sputter target surface, restricts the trajectories of the electrons in the plasma, thereby intensifying and confining the plasma. Ions in the plasma bombard the sputter target dislodging atoms of the target material which are then deposited on a substrate.
In recent years wafer sizes have continually increased, and now the use of eight-inch diameter wafers is common in the industry. Large wafer sizes permit a larger number of integrated circuit devices to be grown on a single substrate. However, larger wafer sizes impose greater demands on sputtering systems. For example, one requirement of a sputtering system used in semiconductor processing is that it deposit a layer of uniform thickness over the entire wafer surface. (Hereinafter the term uniformity will be used in connection with the thickness of the deposited film unless the context suggests otherwise.) Lack of uniformity may result in lowered device yield (i.e., the percentage of devices which meet operating specifications) and/or variations in device performance. Larger wafer sizes make it more difficult to achieve very demanding levels of uniformity. Likewise, the trend towards ever smaller integrated circuit device geometries has required that even greater levels of sputtered film uniformity be achieved.
Other sputtered film characteristics are also quite important to integrated circuit device manufacturers. For example, as noted above, sputtered conductive material is frequently used to form interconnects between device layers. Forming interconnects involves uniformly filling small diameter holes, called vias, in the surface of the wafer. As integrated circuit device geometries have shrunk, the difficulty in filling vias with sputtered material has increased appreciably. Step coverage, or the ability of the sputtered film to evenly conform to angular features on the wafer surface is, likewise, another important film characteristic.
An earlier approach to improving the uniformity and step coverage characteristics of a sputtering system is to sputter from two concentric targets. For an example of this approach see U.S. Pat. No. 4,606,806 which describes a sputtering source sold by the assignee of the present invention under the trademark ConMag.RTM. II. In the ConMag.RTM. II sputtering source each of the sputter targets has a unique shape and its own separate power supply enabling separate control over the sputtering rate from each target.
A number of commercially available sputtering sources use planar sputtering targets. Early designs, wherein the planar magnetron sputtering device used a stationary magnet had practical shortcomings, the most serious of which is that the plasma discharge is localized and erodes a narrow groove in the target in the vicinity of the greatest magnetic field strength. This localized erosion generates a non-uniform distribution of sputtered atoms from the target and a film with non-uniform thickness on the semiconductor wafer. The non-uniform erosion of the sputter target leads to inefficient target utilization. Given the high cost of the sputter targets used in semiconductor manufacture, it is important to obtain the greatest possible target utilization that is consistent with the need for sputtered film uniformity and other required sputtered film characteristics.
Numerous attempts, some partially successful, have been made to modify the planar magnetron source to extend the target erosion and to make the distribution of sputtered atoms more uniform. Attempts have been made to spread out the erosion over a larger surface area using extended magnetic fields. The magnets required for such an approach are large and complicated, and it is difficult to assure that the properties of the magnetron do not change as the target erodes away. The resulting erosion pattern is thus difficult to predict.
U.S. Pat. No. 4,444,643, which is incorporated herein by reference, describes a sputtering device which includes a mechanically rotated annular permanent magnet assembly. The rotation of the permanent magnet assembly causes erosion over a wider area of the target. A version of the sputtering source described in the '643 patent has been sold commercially by the assignee of the present invention under the trademark VersaMag.TM.. This source relies on a rotating magnet mounted behind the target for moving the plasma over the face of the target. Rotation of the plasma was introduced for the purposes of improving uniformity and step coverage, as well as improving the uniformity of target erosion so that targets are more efficiently utilized.
The VersaMag sputtering source, while a significant improvement over planar magnetron sources employing stationary magnets, nonetheless did not produce truly uniform sputtered film nor uniform target utilization. Thus, efforts have been made to develop improved rotating magnet designs for use with planar targets. (The term "planar target" is intended throughout this specification to be descriptive of the sputter target surface before it is eroded. Those skilled in the art will recognize that after the target has been eroded it may no longer have a planar surface.)
One direction that has been taken by those seeking to improve the design of rotating magnets used with planar magnetron sputtering sources has been the used of closed-loop, generally heart-shaped magnet configurations. Such magnet configurations typically employ an array of magnets which are centered along a line defining a heart-shaped, closed loop.
One such arrangement is described in U.S. Pat. No. 4,872,964, entitled "Planar Magnetron Sputtering Apparatus And Its Magnetic Source", issued Oct. 10, 1989 to Suzuki, et al. Suzuki, et al., review the shortcomings of a sputtering source of the type described in the '643 patent and describe a heart-shaped rotating magnet array which is said to produce more uniform target erosion. However, the Suzuki, et al., patent overly simplifies the mathematics of the situation and, therefore, does not fully teach how to obtain truly uniform target erosion. In apparent recognition of this shortcoming, Suzuki, et al., describe the need to adjust the magnet array, after it has been laid out in accordance with their mathematical analysis, "to get more uniform erosion after a test run of the sputtering apparatus." (Col. 5, lines 27-28.) Unfortunately, Suzuki, et al., do not teach any methodology for making the necessary adjustments. The teachings of Suzuki, et al., are directed to how to obtain uniform erosion of the target. While uniform target erosion is important, the characteristics of the sputtered film, such as uniformity, are of greater importance to integrated circuit device manufacturers. In many instances, as will be described below, a non-uniform target erosion pattern improves the uniformity of the sputtered film.
Another sputtering source having a heart-shaped magnet arrangement is described in Japanese Patent Application Publication (Kokai) No. 62-211,375 entitled "Sputtering Apparatus", published Sept. 17, 1987. That patent prescribes the use of a heart-shaped closed loop magnet having a curve defined by the equation r=l-a+2a.vertline..theta..vertline./.pi., (for -.pi.23 .theta..ltoreq..pi.); where the center of the sputter target is located at the origin of a polar coordinate system, r is the distance between the origin and a point on the curve defining the magnet centerline, l is the distance between the center of the heart and the cusp of the heart, and a is the distance between the center of the heart and the center of the target. No derivation is given as to how the inventors arrived at this equation, and it appears to be a compromise between the annular-shaped magnet of the '643 patent and the heart-shaped magnet of the '764 patent. As discussed in the '375 application, a magnet having the prescribed curve will not produce uniform erosion. Moreover, the '375 application does not teach how to obtain any arbitrarily selected erosion profile.
U.S. Pat. No. 4,995,958, entitled "Sputtering Apparatus With A Rotating Magnet Array Having A Geometry For Specified Target Erosion Profile", issued Feb. 26, 1991, to Anderson, et al., also assigned to the assignee of the present invention, describes another generally heart-shaped, closed-loop magnet array for use in a planar magnetron sputtering source. The Anderson, et al., patent, which is hereby incorporated by reference, includes a rigorous mathematical analysis to show how to construct a closed-loop rotating magnet to realize a predetermined erosion profile to thereby achieve, for example, highly efficient target material utilization and high deposition rates. It is noted that the invention of the '958 patent is readily adapted to use in a VersaMag.TM. sputtering source.
Among other things, the '958 patent describes the shortcomings of the aforementioned Suzuki, et al., patent and the teaching of the '375 Japanese patent application, showing how each reference fails to provide a teaching which truly enables one to obtain uniform erosion of a planar sputter target. Importantly, FIGS. 12A-12E of the '958 patent, and the related text, clearly show that minor changes in the shape of a heart-shaped magnet may lead to very dramatic differences in the resulting erosion pattern of the sputter target. (It is believed that this is also shown by the '375 application.) Given the demonstrated fact that minor perturbations of the shape of a heart-shaped magnet may cause significant changes in the resulting target erosion profile, it becomes quite difficult to optimize the shape empirically. Thus, Anderson, et al.'s, mathematical analysis is a highly significant teaching in making heart-shaped, closed-loop magnets practically useful.
A closed-loop magnet configuration of the type described in the '958 patent has the additional advantage of being easily adjustable so that the shape of the magnet array, and therefore the characteristics of the sputtering source, can be changed without great difficulty or expense. As described in that patent, a plurality of magnets are held in position by two iron keepers, or pole pieces, which define the shape of the closed loop. Replacement and/or adjustment of the iron keepers to provide a different closed-loop configuration is a relatively simple matter. In this manner it is possible to use one source for different purposes, or to adjust the source as needs change.
A prime objective of the closed-loop rotating magnet of the '958 patent was to achieve better target utilization efficiency, normally an important objective given the high cost of sputter targets, and to achieve high deposition rates, another important factor due to the demand for ever greater system "throughput". As noted above, the need for greater sputtered film uniformity generally outweighs the need for efficient target utilization and deposition rate. Accordingly, the Anderson, et al., patent provides the basis for obtaining any arbitrary target erosion profile. It is noted, however, that the Anderson et al., patent provides no instruction as to how to determine what erosion profile to use under a given set of conditions to maximize sputtered film uniformity or other sputtered film characteristics.
As described therein, the mathematical analysis provided by Anderson, et al., is inapplicable at two areas of the heart, i.e., in the area near the "tip" of the heart, which is defined herein to mean the generally convex portion of the loop farthest away from the axis of rotation, and in the area near the "cusp" of the heart, which is defined herein to mean the generally concave portion nearest the axis of rotation and which lies between the two lobe-shaped portions of the heart. As a result of the inapplicability of the Anderson, et al., teaching to the region of the cusp of the heart, the designs they show leave the very center of the target unused, and are not optimized for best utilization of the sputter target edge. Moreover, the analysis of the '958 patent is based on the assumption that the magnet has uniform strength at all point along the loop, i.e., the sputtering intensity is the same at all points. In other words, the total quantity of material sputtered per unit length of the magnet is a constant. It has been observed that this assumption is not correct.
It is noted that all of the heart-shaped designs shown by Anderson, et al., Suzuki, et al., and the '375 application are symmetrical about a line which passes through the tip, the cusp, and the axis of rotation of the heart. The symmetry of the Anderson, et al., designs is due to the fact that their method of generating a heart-shaped magnet is by forming a spiral-like shape over 180.degree. (i.e., over one half of a polar coordinate system) and then mirroring this shape to close the loop over the remaining 180.degree.. However, as used herein, the term heart-shaped does not require that there be two strictly symmetrical halves. As will be described below, there may be circumstances when an asymmetrical heart-shaped magnet is desired. Likewise, as used herein, the term heart-shaped, does not require that the heart have a noticeable "tip". It has been found that there are advantages to using a design wherein the region farthest from the axis of rotation and generally opposite the cusp forms an arc of a circle. As used herein, the term "heart" implies that there is a cusp-like transition between two lobes. The cusp-like transition may be smoothed for design convenience.
Finally, it has been discovered that there are discrepancies between the position of the magnetic field adjacent to the sputter target surface as predicated by Anderson, et al., and as empirically measured. As noted above, even minor changes in the shape of the magnetic field generated by a heart-shaped magnet may result in significant variations in the erosion profile obtained.